1. Field of the Invention
The present invention relates to acoustic actuators, and more specifically, to actuators for structural and airborne sound generation and active acoustic and vibration control.
2. Description of the Related Art
There are two methods to control unwanted sound and vibration in structures. The first is passive control, and involves adding mass, stiffness, or damping to the structure. This first method is best suited to applications where the frequency band of the disturbance is above 1 or 2 kilohertz. The second method, known as active control, is based upon destructive interference of the sound or vibration field. In active control, a sensor/actuator combination, which is located on the surface of the vibrating structure, is used to detect and to suppress the disturbance. After sensing the disturbance signal, which may be acoustic or vibration or a combination thereof, the active control system reconfigures and conditions the signal, and drives the actuator such that the output field has the same magnitude but opposite phase as the disturbance.
The sensor and electronic subsystems in active vibration and acoustic control systems are more technically advanced than actuator components. Control systems have benefitted from faster and cheaper microelectronics. Similarly, a wide variety of sensors have been developed including optical sensors, piezopolymers, piezocomposites, and acoustic pressure sensors. Because of the wide variety of sensors available, sensor selections may now be based on application specific needs.
There is a pressing need for improvements in available actuator technology. Typically, the weakest link in most active control systems is in the actuator technology. Although actuator devices for underwater systems have been advanced, the use of such devices in-air has been limited by the characteristic impedance load mismatch between the device and the air medium (the impedance load of water is 3700 times higher than that of air). Consequently, the displacements of the in-air actuators must be much greater than the displacements of in-water actuators, in order to realize the same degree of improvement in acoustic suppression.
An in-air actuator which exhibits a large displacement at low frequency and has a linear near-field velocity (displacement) profile is urgently needed for applications such as structural active acoustic suppression and in-air active acoustic suppression. Other features which are desired include low weight, thin geometry, and low electrical impedance. Because many active control systems are in environments which require them to be configured as large sheets or panels, such as large vibrating machinery mounts on power plant type conditions, they must be rugged enough to withstand rigorous treatment.
Many active control systems utilize either hydraulics or large, heavy electromagnetic force transducers as the actuator component, which are unsuitable for applications requiring lightweight actuators. These technologies may often be constrained by packaging limitations as well as high cost.
In recent years, piezoelectric materials either in the form of piezoceramic-polymer composites, multilayer stacks, or in bender type configurations have been studied as the actuator components in active control applications. Multilayer stacks and peizoceramic-polymer composites are characterized as generating high force and low displacement, whereas the flexors exhibit low force and high displacement capabilities.
An example is described in U.S. Pat. No. 6,349,141, Robert Corsaro, Light Weight Polymeric Sound Generator. This approach uses 4 layers of piezoelectric or electrostrictive film configured as a dual bi-laminate bender. The top and bottom bilaminates are separately formed in a precurved press to form a rippled geometry, then are attached back to back, and optional flat cover plates are applied. Application of voltage to the bilaminates generates a net thickness change, resulting in displacement of the surface and a corresponding sound pressure level change.
Another example of an electrostrictive polymer film (EPF) based in-air acoustic projector is described in xe2x80x9cAcoustic Performance of an Electrostrictive Polymer Film Loudspeakerxe2x80x9d, Richard Heydt, Ron Pelrine, Jose Joseph, Joseph Eckerle, and Roy Kornbluh, J. Acoustic Soc. Am. 107(2), February 2000, 833-839. The projector demonstrated appears to be most effective at relatively higher frequencies of 500-5000 Hz.
A piezoelectric in-air acoustic transducer based on applying a cover plate to two piezoelectric bimorph support structures is described in Baomin Xu, Qiming Zhang, V. D. Kugel and L. E. Cross, xe2x80x9cPiezoelectric Air Transducer for Active Air Controlxe2x80x9d, Smart Structures and Materials 1996: Smart Structures and Integrated Systems, Indirjit Chopra, Editor, Proc. SPIE 2717, 388-398 (1996).
Similarly, Brody D. Johnson and Chris R. Fuller disclose a method of using skin attached to structurally mounted piezoelectric bimorph supports for structural active acoustic control in xe2x80x9cBroadband Control of Plate Radiation Using a Piezoelectric, Double-amplifier Active-skin and Structural Acoustic Sensingxe2x80x9d Brody Johnson and Chris R. Fuller, J. Acoustic Soc. Am. 107(2), February 2000 876-884. The predicted power attenuation is in excess of 10 dB between 250 and 750 Hz.
None of the actuators to date have demonstrated sufficiently high displacement at low frequencies. A lightweight actuator has been developed which has high displacement at low frequencies as described herein.
It is an object of this invention to provide a lightweight, high power, low frequency sound generator useful for active acoustic control of airborne or structure-borne acoustic noise. It is another object of this invention to provide a smart acoustic blanket which can be adhered to a surface to acoustically cancel the undesired structure-borne acoustic noise.
It is another object of this invention to provide a smart acoustic blanket for acoustically canceling undesired airborne noise.
It is another object of this invention to provide small, lightweight high displacement acoustic actuators which produce high power sounds, responsive to electrical signals.
These and other objects are achieved by adhering a polymer membrane to the surface of a piezoelectric driver designed for a desired resonance frequency, and providing electrical signals to the inner and outer surfaces of the piezoelectric driver, producing vibration in the membrane at the desired resonance frequency.